Aggregates, which are essential ingredients of concrete, may be derived from natural sources with minimal processing or from naturally occurring materials that are heat treated. Aggregates may also be synthetic. Natural sources, such as quarries, pits in ground, and riverbeds, for example, are generally composed of rock fragments, gravel, stone, and sand, which may be crushed, washed, and sized for use, as needed. Natural materials that may be used to form aggregates include clay, shale, and slate, which are pyroprocessed, causing expansion of the material. OPTIROC and LECA are examples of commercially available expanded clay aggregates, for example. Synthetic aggregates may comprise industrial byproducts, which may be waste materials. LYTAG, for example, is a commercially available sintered aggregate comprising pulverized fuel ash (“PFA”), also known as fly ash. PFA is produced from the combustion of coal in power plants, for example.
Aggregates may be lightweight or normal weight. Lightweight aggregates (“LWAs”) have a particle density of less than 2.0 g/cm3 or a dry loose bulk density of less than 1.1 g/cm3, as defined in ASTM specification C330. Normal weight aggregates from gravel, sand, and crushed stone, for example, generally have bulk specific gravities of from about 2.4 to about 2.9 (both oven-dry and saturated-surface-dry), and bulk densities of up to about 1.7 g/cm3. High quality LWAs have a strong but low density porous sintered ceramic core of uniform structural strength and a dense, continuous, relatively impermeable surface layer to inhibit water absorption. They are physically stable, durable, and environmentally inert. LWAs should also be nearly spherical, to improve concrete properties and provide good adherence to concrete paste. Suitable sizes for incorporation in concrete are in a range of from about 4.75 mm to about 25 mm, or 2.36 mm to 9.5 mm for coarse aggregates, in accordance with ASTM Specification C330. Smaller, fine aggregates, which are a byproduct of LWA production, may also be used, to replace sand in concrete, for example. For use in concrete, LWAs should have a sufficient crushing strength and resistance to fragmentation so that the resulting concrete has a strength of greater than 10 MPa and a dry density in a range of about 1.5 g/cm3 to about 2.0 g/cm3. Concrete containing LWAs (“LWA concrete”) may also have a density as low as about 300 kg/m3.
While LWA concrete may be 20-30% lighter than conventional concrete, it may be just as strong. Even when it is not as strong as conventional concrete, the LWA concrete may have reduced structural dead loads enabling the use of longer spans, narrower cross-sections, and reduced reinforcement in structures. The lower weight of the LWA concrete facilitates handling and reduces transport, equipment, and manpower costs. LWA concrete may be particularly useful in construction slabs in high rise buildings and in concrete arch bridges, for example. LWA concrete may also have improved insulating properties, freeze-thaw performance, fire resistance, and sound reduction. LWAs can also be used in the construction of other structures, in highways, and as soil fillers, for example.
Quarrying is the largest source of aggregates by volume in most countries. Despite the many advantages of LWAs, aggregate extraction is complicated by environmental and legal issues, availability, and transportation and other costs, for example.
Waste disposal is another area presenting significant environmental and legal issues. Due to the exhaustion of available landfill sites, the difficulties in acquiring new sites, the adverse environmental effects, and the costs of landfilling, disposal of waste materials has been a significant problem for many years. For example, incinerator bottom ash (“IBA”), which is a heavy ash stream generated from municipal solid waste (“MSW”) incineration, is a significant waste in terms of volume. IBA accounts for about 75% to about 80% of the total weight of MSW incinerator residues. IBA comprises a heterogeneous mixture of slag, glass, ceramics, ferrous and nonferrous metals, minerals, other non-combustibles, and unburnt organic matter. The considerable amounts of IBA produced present significant disposal problems. When landfilled, heavy metals may leach from the IBA into the ground and underground resources. IBA is currently used in its raw form (without heat treatment) in the construction of fills and embankments, pavement base and road sub-base courses, soil stabilization, landfill cover, in bricks, blocks, and paving stones, and as fillers in particular applications. Although considered a relatively inert waste, leaching of heavy metals in these applications is possible. Concrete containing IBA is weaker than concrete incorporating LYTAG, for example. The IBA may also chemically react with cement, leading to swelling and cracking.
Significant volumes of waste are also produced by electricity-generating coal power plants, mainly in the form of fine-grained particulate material in flue gases from the power plant furnace, which is referred to as pulverized fuel ash (“PFA”). PFA accounts for 70 to 80% of the coal ash produced. As mentioned above, sintered LWAs comprising PFA are commercially available under the tradename LYTAG. ASTM C 618 defines two major classes of PFA, on the basis of their chemical composition, Class F and Class C. Class F PFA, which comprises siliceous and sometimes aluminous material, is normally produced from burning anthracite or bituminous coal and has little or no cementitious value. Class C PFA, which is normally produced from the burning of subbituminous coal and lignite, usually contains significant amount of calcium hydroxide (CaOH) or lime (CaO). Class C PFA has some cementitious properties. The majority of PFA produced is currently disposed in landfills at a great cost and risk of leakage of heavy metals that could contaminate underground aquifers.
In addition to PFA, power plants produce furnace bottom ash (“FBA”), which is a heavier, coarse ash material that falls through the bottom of the furnace. FBA is classified as either wet or dry bottom ash, depending on the type of boiler used. Although both coal combustion by-products have properties that make them desirable for use in a range of applications, more than 70% of the coal ash is unused. The majority of it is disposed of in landfills. FBA is currently used in its raw form as an aggregate in lightweight concrete masonry units, as raw feed material in Portland cement, as a road base and subbase aggregate, as a structural fill material (such as embankments and retaining walls), and as a fine aggregate in asphalt paving.
The mining industry produces significant quantities of waste in the form of powder, mud, and crushed material of different sizes, generated during crushing and washing operations. About 75 percent of these residues contain various types of soft stones such as marble, china, and travertine. The remainder of the residues contain hard stones, such as granite.
Granite sawmills and granite cutting machines used in granite mining, for example, which is one of the most important mining sectors, generate a large amount of granite waste residues in the form of powder or mud from sawing and washing processes. Such waste needs to be treated prior to lagoon or landfill disposal in order to prevent contamination of ground or underground water aquifers. Granite comprises silica and alumina.
Waste glass, which is removed from the MSW stream, is another waste product. Waste glass has been used in highway construction as an aggregate substitute in asphalt paving and as a granular base or fill material. Waste glass varies in sizes from between about 25 mm to about 100 mm.
U.S. Pat. No. 4,120,735 to Smith discloses a method of producing a brick or similarly fired construction unit, such as a ceramic-type tile or vitrified pipe, comprising mixing at least 50% by weight inorganic, non-ferrous residue from municipal incinerators (which generally refers to incinerator bottom ash) with coal fly ash and a binder, such as sodium silicate. The mixture is shaped and then fired in three steps. First, the mixture is heated at 180° C. for one hour, to ensure that moisture in the mixture is evaporated. Then the mixture is heated in increments of 65° C./minute to 550° C. and held overnight, to burn off carbon. Then the mixture is fired at temperatures of from about 1,700° F. (926° C.) to about 2,000° F. (1,093° C.), to form the brick. Smith emphasizes that the addition of the incinerator residue to the coal fly ash lowered the firing temperature as compared to a coal fly ash brick. Smith states that the incinerator residue, instead of the fly ash, melts to produce bonding on cooling. Considerable fusion is said to take place between 1,700° F. (926° C.) and 1,750° C. (954° C.). Smith also reports better brick properties as the proportion of incinerator residues increase. A preferred composition is therefore 50% to 60% incinerator residue 1% to 4% binder, and the remainder coal fly ash. Based on the low firing temperature, it is believed that the incinerator residue comprised predominantly glass and possibly incinerator fly ash. Due to the reported strengths, it is also believed that Smith produced a vitrified brick, with a large glassy, amorphous phase. The brick has high strength and low porosity, as the melted glass components of the incinerator residue filled most pores.
The economic burdens and the risks of waste disposal make it advantageous to develop alternative techniques for converting wastes into revenue-earning products, which would also reduce the demand for less accessible, non-renewable materials.